Light emitting devices are used in many applications including optical communication systems. Optical communication systems continue to increase in use due to the large bandwidth they provide for transporting information signals such as voice and data signals. Optical communication systems provide a broad bandwidth and high speed and are suitable for efficiently communicating information signals at high data rates over long distances. Over long distances, optical communication systems operate at relatively long wavelengths, in the order of 1.3 micrometers (μm) to 1.55 μm, because optical fibers generally have their lowest attenuation in this wavelength range. These long-wavelength optical communication systems have one or more light sources capable of emitting light at a wavelength in this relatively long wavelength range. In many optical communication systems, the light source is a vertical-cavity surface-emitting laser (VCSEL), although other types of light sources can alternatively be used.
VCSELs operating at the long wavelengths used for optical fiber communication have an active region composed in part of one or more quantum well layers of indium gallium arsenide nitride (InGaAsN). This material allows the operating wavelength of the VCSEL to be extended to approximately 1.3 μm. Furthermore, it is predicted that active regions composed in part of quantum well layers of InGaAsN will provide excellent performance characteristics in other applications using photonic devices, such as light emitting diodes (LEDs), edge-emitting lasers, and vertical-cavity surface-emitting lasers (VCSELs). These excellent performance characteristics are a consequence of the strong electron confinement provided by AlGaAs heterostructures that provide carrier and optical confinement in both edge-emitting and surface-emitting devices. In both edge-emitting and surface-emitting lasers, active regions incorporating quantum well layers of InGaAsN provide performance benefits. InGaAsN therefore has the potential to be a viable substitute for indium gallium arsenide phosphide (InGaAsP) in 1.3 μm lasers.
Molecular beam epitaxy (MBE) is conventionally used to form layers of InGaAsN. MBE uses a nitrogen radical generated by a plasma source as a source of active nitrogen species. The purity of the nitrogen provided by MBE is typically high because high-purity nitrogen gas is widely available. Further, using MBE, the incorporation efficiency of nitrogen into the epitaxial layer approaches unity. However, MBE suffers from a low growth rate, resulting in a long growth time. Moreover, MBE does not scale well, and therefore does not lend itself to high-volume production of light emitting devices.
Another technique for producing semiconductor-based light emitting devices is known as organometallic vapor phase epitaxy (OMVPE), sometimes referred to as metal organic chemical vapor deposition (MOCVD). In OMVPE, a carrier gas is passed through liquid chemical precursors to generate respective chemical vapors that are passed over a heated semiconductor substrate located in a reactor. Conditions in the reactor are controlled so that atomic species released from the chemical vapors passing over the substrate bond to the surface of the substrate to form an epitaxial film.
High quality films of InGaAsN are difficult to grow using OMVPE because the purity of the nitrogen precursor is difficult to control. A typical nitrogen precursor is dimethylhydrazine (DMHy), [CH3]2NNH2,. Moreover, the nitrides that partially constitute InGaAsN are somewhat immiscible with the remaining constituents. These factors cause the distribution of nitrogen in the layer to be non-uniform. This results in bandgap fluctuations throughout the layer. The bandgap fluctuations broaden the spontaneous emission spectrum and the gain spectrum, which raises threshold current of the laser.
Furthermore, DMHy does not readily decompose to yield atomic nitrogen. This makes it difficult to grow InGaAsN with a nitrogen fraction sufficient to reduce the band gap of the material to one corresponding to emission at 1.3 μm. This nitrogen fraction is typically in a design range extending from about 0.2% to about 5%. To achieve a nitrogen fraction greater than a fraction of 1%, the ratio of DMHy to the arsenic precursor, typically arsine (AsH3), is conventionally increased relative to the expected ratio because the arsenic provided by the arsenic precursor competes with the nitrogen released from the DMHy for the available group-V lattice sites. However, reducing the flow of the arsenic precursor tends to reduce the optical quality of the InGaAsN film.
To grow a high-quality InGaAsN layer with a sufficient nitrogen fraction using OMVPE, an extremely high ratio of the dimethylhydrazine flow rate to the total Group V precursor flow rate is used. However, even when the DMHy ratio is raised to 90% or greater, a negligible nitrogen fraction (<<1%) may be obtained, despite the high nitrogen content of the vapor.
The presence of indium in the grown material makes the incorporation of nitrogen even more problematic. Indium-containing material is a necessary component of the quantum well layers of a 1.3 μm laser diode. Ideally, for 1.3 μm light emitting devices, the indium fraction should be greater than or equal to about 30%, and the nitrogen fraction should in the above-mentioned design range, i.e., from about 0.2% to about 5%. The longest wavelength that can typically be attained using InGaAs quantum well layers having a maximum indium fraction is about 1.2 μm. The maximum indium fraction is limited by biaxial compression. Using InGaAsN with a relatively small nitrogen fraction in the above-mentioned design range as the material of the quantum well layers reduces the bandgap energy into a range that extends the wavelength range to 1.3 μm and beyond. Nevertheless, a nitrogen content of even 1% is difficult to attain using conventional OMVPE, even when the nitrogen precursor exceeds 90% of the total Group V precursors.
Therefore, what is needed is a method in which OMVPE is used to mass produce a high optical quality light emitting device having InGaAsN as the material of its active layer(s).